I would think that a heli-coil repair could be done right on the tractor. Make sure you drill square...maybe a drill jig that sits flat on the casting would be in order. But maybe someone else on the board has some experience with this.

Charles- On high end stuff like aircraft heli-coils are installed on parts when they are new. It makes the threads more durable and strong then they would otherwise be in the base material. My own experience is they not only provide a good repair butt the repair may be better than it was originally, if done correctly.